The invention concerns a method of recovering cobalt oxalate and/or manganese oxalate from solutions containing cobalt and/or manganese in aqueous acetic acid or in organic solvents or solvent mixtures or extracts which are at least partially miscible with water, by the addition to the solutions or extracts of sodium oxalate, potassium oxalate or ammonium oxalate per mole of dissolved cobalt compound and/or manganese compound.
The method of the invention is particularly suitable for the recovery of cobalt and/or manganese catalysts from acetic acid mother liquors from oxidation processes for the preparation of aromatic carboxylic acids by the oxidation of alkyl aromatics with atmospheric oxygen in acetic acid solution. By oxidation processes such as these, terephthalic acid, for example, is prepared from p-xylene, p-nitrobenzoic acid is prepared from p-nitrotoluene, and other aromatic carboxylic acids are prepared from the corresponding alkyl aromatics.
The recovery of the catalyst is an important factor in the economy of such processes. In most of the methods described for the recovery of catalyst from oxidation mother liquors, such as the one described in DE-OS No. 2,131,470, DE-OS No. 2,260,491 or DE-OS No. 2,419,323, the catalysts are isolated in carbonate form. For this purpose the acetic acid solutions must first be concentrated by evaporation and the residues extracted with water. The extraction of this often tarry, occasionally even two-phase residue, is problematical and not always sufficiently effective. An incomplete extraction of the cobalt, however, signifies not only losses of the relatively expensive catalyst, but it also makes it considerably more difficult to dispose of the residual product still containing cobalt.
A method for the recovery of cobalt and/or manganese catalyst from acetic acid oxidation mother liquors is described in Japanese OS 9,7593/76: the heavy metal catalyst in the oxidation mother liquor is precipitated by the addition of oxalic acid the form of an oxalate of poor solubility, and isolated by solid-from-liquid separation methods. This method offers the possibility of separating other heavy metal oxalates precipitated together with the cobalt and/or manganese oxalate by washing them out.
When the method of Japanese OS 97,593/76 was practiced, it was found that the cobalt oxalate which precipitated from cobalt-containing acetic acid oxidation mother liquors or from acetic acid cobalt acetate solutions is virtually unfiltrable. Even when the precipitation conditions were altered as regards temperature, degree of aqueous dilution of the solution, cobalt concentration or the way in which the oxalic acid was added, ease of filtrability could not be achieved. The addition of filter aids brought no improvement. In all cases, the cobalt and/or manganese oxalate was obtained in a form in which it was virtually unfiltrable with filtration centrifuges or suction or pressure filters.
The reason for the above-described filtration problems in a cobalt oxalate precipitated in accordance with Japanese OS No. 97,593/76 became evident when the product was subjected to examination in the scanning electron microscope: the crystallizate consisted of small, rounded particles of a diameter of only about 0.1 to 0.3 microns. The filtration of such a finely crystalline product is extremely problematical and is impossible using common filtration apparatus such as filtration centrifuges, suction filters or pressure filters under technical conditions, since even in thicknesses of less than 0.5 cm, the fine crystallizate was compressed to a very dense layer that blocked passage of the liquid.